Conventionally, as a technique in this field, a position detecting device disclosed in Japanese Unexamined Patent Application Publication No. 2003-337029 is known. In this device, the relative position relationship between a host vehicle and another vehicle is calculated on the basis of host vehicle position information by a GPS, which is created by the host vehicle, and the other vehicle position information by a GPS received from another vehicle. In addition, it is possible to know the traveling position of another vehicle by matching and specifying the host vehicle position and the other vehicle position on the read map while maintaining this positional relationship.